This invention relates generally to devices for gasification of waste water and to waste water treatment plants which includes such gasification devices, including particularly devices for oxygenation of waste water.
Biological waste water treatment plants or the like provide treatment vessels wherein the natural processes of purification are accelerated by the mechanical introduction of an oxygen supply. Either pure oxygen or oxygen provided via air can be so introduced. Although approximately five times more air is required than oxygen to obtain the same degree of purification, the cost of industrial oxygen indeed favours the use of air in many such systems.
In order to provide sufficient oxygen from the air to enable the higher process rates it is necessary to introduce the air by means which function to increase the efficiency of gas transfer into the waste water. The devices thus utilized, whether introducing air or oxygen, also must function to enable thorough mixing, so that the reactants may be brought into intimate contact and without leaving dead areas of untreated liquid in the treatment vessel.
Several types of apparatus have been employed over the years for introducing air into liquids for the aforementioned purposes. Among these are surface aeration devices, which mechanically whip the waste liquor surface, to thereby expose more liquid surface to the surrounding air.
In a second type of aeration apparatus air is injected under pressure into the liquid body, to produce a mass of either small or large bubbles.
In another type of aeration apparatus, liquid under pressure is pumped from the bottom of a vessel and redistributed through a fine spray onto the liquid surface of the same vessel. This type of device in its mode of operation, is generally similar to surface aeration devices.
The present invention is particularly applicable to the aforementioned technology wherein air is injected into the body of the liquid to be treated, as by means of spargers, bubblers, or similar gas dispensers. In considering the mechanism of oxygen uptake or transfer in this environment, analysis by the inventor indicates that the principal factors encouraging maximum oxygen uptake from air are: (1) the bubble diameter should not be too large; (2) the bubbles injected from the dispenser should be encouraged to contact as much oxygen-starved liquid as possible immediately after emission from the dispenser; and (3) the oxygen-rich liquid immediately above the dispenser should be dispersed into the main body of liquid as rapidly as possible. In terms of these performance criteria, however, it is found that presently available commercial devices are inadequate in one or more such respects.